Generally, the magnetic circuit of an electric machine consists of a piling up assembly of metal laminations. To prevent overheating of the laminations caused by the flow of Eddy currents, the laminations are insulated from each other by means of insulating varnish or other materials. Defects to which reference is herein made are caused by short circuits between the metal laminations. In the case of alternators' stators, such defects can be caused for example by a metallic piece (e.g. a bolt) detaching and causing damages to the face of the stator, a breakdown between the winding under voltage (13 kV) and the stator, or the wear of the varnish. These defects must be located.
One method of the prior art consists in a visual inspection. This basic method is still used and allows to locate some apparent defects. This method is subjective as it depends on the skills of the person carrying it out. It cannot be used to detect defects in the notches or between the metal strips of a stator. This method is also difficult to carry out with a rotor in place.
Another method of the prior art consists in a magnetization test. In this method, an intense magnetic flux .phi.t (.apprxeq.80% of the nominal flux) is generated in the stator ring by means of temporary excitation windings mounted on the stator for this purpose. An infrared camera observes the inner face of the stator to detect the hot spots. This method is qualitative and cannot be used to detect defects on only a portion. It requires a powerful external excitation source, and the excitation windings are important. The installation and test times are long, it requires several workers and is impracticable with the rotor in place.
Another method of the prior art consists in an electromagnetic core imperfection detection test (ELCID). In this method, a low magnetic flux .phi.t (.apprxeq.4% of the nominal flux) is induced in the stator ring by means of a temporary excitation winding mounted on the stator for this purpose. A measurement probe explores the inner face of the stator to detect anomalies. In this method, the preparation of the stator is similar to the magnetization test, except that it involves a less powerful source and a more modest temporary excitation winding. This method is quantitative. Nevertheless, it is more or less suitable to detect defects between the metal laminations, it still requires an external excitation source and the mounting of an excitation winding, and several workers. Furthermore, the installation time is long and the test time is in the average.
Both last mentioned methods involve high costs, and may provide inaccurate results.